Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.

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Abstract. Carbon dioxide and methane fluxes were measured at a tundra site near Chokurdakh, in the lowlands of the Indigirka river in north-east Siberia. This site is one of the few stations on Russian tundra and it is different from most other tundra flux stations in its continentality. A suite of methods was applied to determine the fluxes of NEE, GPP, R eco and methane, including eddy covariance, chambers and leaf cuvettes. Net carbon dioxide fluxes were high compared with other tundra sites, with NEE=−92 g C m −2 yr −1 , which is composed of an R eco =+141 g C m −2 yr −1 and GPP=−232 g C m −2 yr −1 . This large carbon dioxide sink may be explained by the continental climate, that is reflected in low winter soil temperatures (−14 • C), reducing the respiration rates, and short, relatively warm summers, stimulating high photosynthesis rates. Interannual variability in GPP was dominated by the frequency of light limitation (R g <200 W m −2 ), whereas R eco depends most directly on soil temperature and time in the growing season, which serves as a proxy of the combined effects of active layer depth, leaf area index, soil moisture and substrate availability. The methane flux, in units of global warming potential, was +28 g C-CO 2 e m −2 yr −1 , so that the greenhouse gas balance was −64 g C-CO 2 e m −2 yr −1 . Methane fluxes depended only slightly on soil temperature and were highly sensitive to hydrological conditions and vegetation composition.

[1] With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (R eco ). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in R eco . Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arctic.

1Roughness length and zero-plane displacement over boreal, cool-and warm-temperate 2 forests were observed and parameterised using forest structure data. Previous mod-3 els for roughness length and zero-plane displacement using leaf area index and 4 frontal area index did not describe intersite differences, and the model for zero-5 plane displacement did not express seasonal variations with the change of leaf area 6 that was smaller in dense forest than in sparse forest. The observed results show 7 that intersite differences of normalised zero-plane displacement were related to stand 8 density, and seasonal variations were related to leaf area index at each forest, with 9 the degree depending on stand density. From these observations, a new concept is 10 proposed for normalised zero-plane displacement: the basal part is primarily de-11 termined by the density of stems and branches (stand density), while the seasonal 12 variation depends on the density of leaves (leaf area index), which is limited to